To sustain rapid cell division and growth, tumors undergo metabolic reprogramming. Understanding how cancer cells adjust to the metabolic needs of their unique biology has been a focus of cancer research for many years (Cairns et al., (2011) Nat. Rev. Cancer 11: 85-95). The concept of metabolic adaptation was first reported by Otto Warburg over 80 years ago, after he observed that cancer cells exhibited enhanced consumption of glucose and production of lactate in comparison to normal tissue, even under aerobic conditions (Warburg O (1956) Science 123: 309-314). This phenomenon became known as the Warburg effect. While normal, differentiated cells maximize ATP production by mitochondrial oxidative phosphorylation of glucose under normoxic conditions, cancer cells produce substantially less ATP from glucose using aerobic glycolysis (Wong et al., (2013) Int. J. Cell Biol. 2013: 242513).
The enhancement in aerobic glycolysis combined with the dynamic processes in cancer cells enables glycolytic intermediates to be redirected for the biosynthetic production of cellular building blocks such as nucleotides, amino acids, and lipids, while still producing ATP (Wong et al., (2013) Int. J. Cell Biol. 2013: 242513), therefore fulfilling the requirements for macromolecular synthesis and energy production (Vander Heiden et al., (2009) Science 324: 1029-1033; Hsu & Sabatini (2008) Cell 134: 703-707). The Warburg effect has been extensively exploited clinically to detect tumors and their response to treatment by [18F]FDG. [18F]FDG PET non-invasively measures rates of glucose metabolism and is approved for the diagnosis of most cancers and has proven particularly useful as a staging and restaging tool that can guide patient care (Kelloff et al., (2005) Clin. Cancer Res. 11: 2785-2808; Sharma et al., (2004) Radiographics 24: 419-434). Since glucose metabolism is an essential cellular process, [18F]FDG is not specific for malignant cells. Particularly in the brain, which has a high rate of glucose metabolism, and therefore a high physiological uptake of [18F]FDG, it is extremely difficult to delineate brain tumor margins with [18F]FDG PET imaging. There is a continuing need for PET imaging agents with selectivity for molecular processes unique to cancer cells.
Pyruvate kinase (PK) catalyzes the final and rate-limiting step in glycolysis, converting phosphoenol pyruvate (PEP) to pyruvate by transferring the high-energy phosphate group to adenosine diphosphate (ADP) and yielding ATP (Wong et al., (2013) Int. J. Cell Biol. 2013: 242513). Reduced PK activity results in a diminished production of pyruvate or prevention of the conversion of glucose to pyruvate, therefore enabling the accumulation of upstream glycolytic intermediates and shifting metabolism towards the anabolic phase. Cancer cells exploit this effect by primarily utilizing the PKM2 isoform of PK, whose activity can be dynamically controlled between the less active PKM2 dimer and the highly active PKM2 tetramer (Christofk et al., (2008) Nature 452: 230-233).
PKM2 is found in most cells with the exception of adult muscle, brain, and liver and is preferentially expressed in all cancers to date (Christofk et al., (2008) Nature 452: 230-233). The dynamic equilibrium between the dimeric and tetrameric states of PKM2 enables proliferating tumor cells to regulate their needs for anabolic and catabolic metabolism. The alternative splicing of PKM2 is regulated by oncogenes c-Myc and HIF-1 (Chaneton & Gottlieb (2012) Trends Biochem. Sci. 37: 309-316), with the quaternary structure of PKM2 regulated by the glycolytic intermediate fructose 1,6-biphosphate (FBP) and growth factor signalling (Bailey et al., (1968) Biochem J. 108: 427-436; Christofk et al., (2008) Nature 452: 181-186).
The expression of PKM2 has been shown to be increased in a diverse range of human cancers, including lung, breast, prostate, blood, cervix, kidney, bladder, and colon, compared to the matched normal tissues (Bluemlein et al., (2011) Oncotarget doi:10.18632/oncotarget.278). PKM2 expression is linked to increased uptake of glucose, enhanced lactate production, and a decrease in oxygen consumption, effects which can be reversed by genetic modifications to replace PKM2 expression with PKM1 (Christofk et al., (2008) Nature 452: 230-233, Luo & Semenza (2012) Trends Endocrinol. Metab. 23: 560-566). The Warburg effect is, therefore, partly mediated by PKM2 expression, with the high expression of dimeric PKM2 in cancer cells contributing to anabolic glucose metabolism, promoting macromolecular biosynthesis and benefiting cancer cell proliferation and growth (Luo & Semenza (2012) Trends Endocrinol. Metab. 23: 560-566).
In recent years, PKM2 has been explored as a potential target for cancer therapy through the development of small molecule activators that promote and stabilize active tetramer formation (Boxer et al., (2010) J. Med. Chem. 53: 1048-1055; lsraelsen et al., (2013) Cell 155: 397-409). Multiple classes of PKM2 activators have been developed, and representative examples are shown in FIG. 6. These examples include a series of N—N′-diaryl sulfonamides (1), thieno[3,2-b]pyrrole[3,2-d]pyridazinones (2, 3), 1-(sulfonyl)-5-(arylsulfonyl)indoline (4), 2-((1H-benzo[d]imidazole-1-methyl)-4H-pyrido[1,2-a]pyrimidin-4-ones (5), 3-(trifluoromethyl)-1H-pyrazole-5-carboxamide (6), and more recently 7-azaindole derivative (7). Clinically relevant imaging agents for the direct, non-invasive measurement of cancer-specific biomarkers are a topic of extreme interest and importance, particularly for brain tumors. Although [18F]FDG PET is approved for use for the diagnosis of most cancers, a high background uptake by surrounding healthy tissue can mask tumor uptake, for example, in the normal brain (Phelps & Mazziotta (1985) Science 228: 799). Consequently, the non-invasive measurement of PKM2 has the potential to play an important role in the detection and management of malignancies where [18F]FDG fails.